The decrease of internal efficiency with increasing temperature in injection lasers has been assumed in the past to be responsible for the increase in threshold and the decrease in external efficiency. This decrease in internal efficiency has been attributed to the greater hole-electron smearing, with a consequent reduction of the degree to which inverted population occurs in the region in which stimulated emission takes place. This paper shows that the increase in hole and electron smearing which occurs at elevated temperatures has a negligible influence in degenerate GaAs laser diodes in contrast to the greater optical loss arising from -region absorption. It is demonstrated that the increase in threshold is due to the stronger -type absorption at 300°K. On this basis, it is also shown that the threshold current ratio K)/ K) ranges from 12 to 20.0. This range is in agreement with experimental values. In this calculation the internal quantum efficiency is assumed to remain constant in the temperature range from 77°K to 300°K. The laser line width increases from 0.75 Å at 77°K to 5 Å at 300°K. In addition, there is a corresponding increase in apparent mode spacing. These data indicate that the cavity decreases by approximately one order of magnitude from 77°K to 300°K. The correspondence between the cavity and the threshold is shown. Calculated and experimental ratio values of threshold and cavity are shown employing absorption data for GaAs in the literature. As further confirmation, curves of threshold, line width, and cavity with temperature are all shown to have the same shape. The data presented and the calculations made on the assumed model show that the -type absorption loss is the dominant mechanism in increasing threshold in degenerate junctions and that hole-electron smearing is negligible.